Blender or CAD software for making a robot/character with swappable parts
Expanding a bit on Duarte Farrajot Ramos' comment above: if you want to engineer routinely, then there are more focussed tools than blender. For the purpose that you describe, blender might suit you fine.

I myself do use blender to prepare engineering models for simulations. Not needing some of the more advanced CAD features, blender turned out to be a practical and very cost effective tool for me with the added advantage that I can model and visualize results using the same tool.

When you want to do precision modelling there are a number of guides and tutorials available on how to tweak blender for that purpose:


Several relatively simple precision tips can be found in this answer: How to model effectively using exact measurements?
An older but very extensive tutorial can be found here: http://www.rab3d.com/tut_blen_precision_guide.php
blendermama has a nice collection of hints: http://blendermama.com/precision-work-in-blender.html
And there are some tutorials on youtube of course: https://www.youtube.com/watch?v=kOxN4CHeBPQ


There are also a number of addons that help: Edge Tools and Caliper from http://www.blender3darchitect.com/ (which also has great tutorials) as well as tinyCAD.